Optical mode filter employing radially asymmetric fiber

Abstract

Fiber amplifier and/or mode filter including a linearly birefringent LMA fiber coiled at a radius of curvature over a bend length to differentiate a fundamental optical mode from supported higher-order modes through bending losses. The LMA fiber may be a polarization-maintaining (PM) fiber having a variety of geometrical core shapes and cladding configurations. In some embodiments, the birefringent LMA fiber includes a radially asymmetric core that is angularly rotated over a length of the coiled fiber to ensure bending losses are experienced in orthogonally oriented higher-order modes associated with some orientation relative to the core orientation. In some embodiments, the fiber coiling is two-dimensional with bending occurring only about one axis. In some embodiments, an asymmetric core is pre-spun to a predetermined axial spin profile. In some embodiments, angular rotation of the core is achieved through mechanically twisting an un-spun fiber over a length of the coil.

What is claimed is: 
     
       1. A fiber amplifier, comprising:
 a light source to produce an optical beam; and 
 a linearly birefringent large mode area (LMA) fiber coupled to the light source to support a fundamental mode and higher-order modes of the optical beam, wherein:
 the LMA fiber has a radially asymmetric core; and 
 
 the asymmetric core has an angular rotation of at least 90° about a fiber axis over a bend length having a radius of curvature about an axis of curvature, non-parallel to the fiber axis. 
 
     
     
       2. The fiber amplifier of  claim 1 , wherein:
 the axis of curvature is orthogonal to the fiber axis over the bend length; and 
 the angular rotation over the bend length has a predetermined spin profile providing a suppression of at least 10 dB/meter of bend length for all higher-order modes. 
 
     
     
       3. The fiber amplifier of  claim 2 , wherein the LMA fiber is coiled about the axis of curvature with a fixed radius of curvature over the bend length and a rate of the angular rotation as a function of fiber length varies over the bend length. 
     
     
       4. The fiber amplifier of  claim 1 , wherein:
 the V-number of the LMA fiber is at least 4 and less than 20; and 
 the core is elliptical with a major axis that is at least 5% larger than a minor axis over the entire bend length. 
 
     
     
       5. The fiber amplifier of  claim 4 , wherein:
 the major axis is oriented substantially parallel to the axis of curvature over a first portion of the bend length and substantially orthogonal to the axis of curvature over a second portion of the bend length. 
 
     
     
       6. The fiber amplifier of  claim 5 , wherein:
 a modal suppression coefficient associated with a first higher mode over the first portion of the bend length is less than a modal suppression coefficient associated with a second higher mode over the second portion of the bend length, and 
 the first portion of the bend length is longer than the second portion of the bend length. 
 
     
     
       7. The fiber amplifier of  claim 1 , wherein:
 the LMA fiber is an active fiber doped with one or more rare-earth element over the bend length; and 
 the angular rotation and radius of curvature over the bend length induces attenuation in the higher-order orthogonal modes that exceeds gain in the higher-order orthogonal modes. 
 
     
     
       8. The fiber amplifier of  claim 1 , wherein the LMA fiber is a polarization maintaining fiber further comprising stress rods in the cladding. 
     
     
       9. The fiber amplifier of  claim 1 , further comprising a mandrel upon which the LMA fiber is wound, wherein the mandrel has a longitudinal axis parallel to the axis of curvature and a radius of curvature about the longitudinal axes that defines the radius of curvature over the bend length. 
     
     
       10. The fiber amplifier of  claim 1 , wherein the LMA fiber comprises a pre-spun core having a predetermined spin profile over the fiber length. 
     
     
       11. A method of fabricating an optical mode filter that attenuates higher-order modes more than the fundamental mode through bend losses, the method comprising:
 receiving a mandrel having a longitudinal axis and a radius of curvature about the longitudinal axis; and 
 winding onto the mandrel a linearly birefringent large mode area (LMA) fiber with a radially asymmetric core operable to support a fundamental mode and higher-order modes of an optical beam, wherein the winding comprises mechanically twisting the LMA fiber to achieve at least 90° of angular rotation about the fiber axis over a bend length having a radius of curvature defined by the mandrel. 
 
     
     
       12. The method of  claim 11 , wherein the twisting defines a spin profile over the bend length that provides a suppression of at least 10 dB/meter of bend length for all higher-order modes. 
     
     
       13. The method of  claim 11 , wherein the twisting is varied over the bend length. 
     
     
       14. The method of  claim 11 , wherein the winding further comprises winding a major axis of the core oriented substantially parallel to the longitudinal mandrel axis over a first portion of the bend length, and winding the major axis substantially orthogonal to the longitudinal mandrel axis over a second portion of the bend length. 
     
     
       15. The method of  claim 14 , wherein:
 modal suppression associated with a first higher mode over the first portion of the bend length is less than modal suppression associated with a second higher mode over the second portion of the bend length, and 
 the first portion of the bend length is longer than the second portion of the bend length. 
 
     
     
       16. The method of  claim 11 , wherein:
 the V-number of the LMA fiber is at least 4 and less than 20; and 
 the core is elliptical with a major axis that is at least 5% larger than a minor axis over the entire bend length. 
 
     
     
       17. The method of  claim 11 , wherein:
 the LMA fiber is an active fiber doped with one or more rare-earth element over the bend length; and 
 the twisting is to angularly rotate the core over the bend length to induce attenuation in the higher-order orthogonal modes that exceeds gain in the higher-order orthogonal modes. 
 
     
     
       18. An optical fiber mode filter, comprising:
 a mandrel having a longitudinal axis and a radius of curvature about the longitudinal axis; 
 a linearly birefringent large mode area (LMA) active fiber with a radially asymmetric core operable to support a fundamental mode and higher-order modes of an optical beam coiled around the mandrel, wherein the fiber is mechanically twisted to angularly rotate the core at least 90° about the fiber axis over a bend length having a radius of curvature defined by the mandrel. 
 
     
     
       19. The mode filter of  claim 18 , wherein:
 the V-number of the LMA fiber is at least 4 and less than 20; 
 the core is elliptical with a major axis that is at least 5% larger than a minor axis over the entire bend length; 
 the major axis is oriented substantially parallel to the longitudinal mandrel axis over a first portion of the bend length and substantially orthogonal to the longitudinal mandrel axis over a second portion of the bend length. 
 
     
     
       20. The mode filter of  claim 19 , wherein:
 a modal suppression coefficient associated with a first higher mode over the first portion of the bend length is less than a modal suppression coefficient associated with a second higher mode over the second portion of the bend length, and 
 the first portion of the bend length is longer than the second portion of the bend length.